<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        body{
            padding: 0;
            margin: 0;
        }
        .father{
            width: 300px;
            height: 300px;
            background-color: pink;
            border: 10px solid yellow;
            padding: 20px;
            margin: 20px;
            position: relative;
        }
        .son{
            width: 100px;
            height: 100px;
            background-color: grey;
            border: 10px solid blue;
            background-color: grey;
            position: absolute;
            /*【1】定位的参照系是父元素的边框内侧。*/
            margin: 0;
            left: 0;
            top: 0;
        }
    </style>
</head>
<body>
<div class="father">
    <div class="son">
        123
    </div>
</div>
</body>
</html>